Radio communication has been in use for many years. Radio communication takes advantage of the phenomenon that radio waves, that is, energy from a certain range of the electromagnetic energy spectrum, may travel for a relatively great distance. In addition, radio waves may be generated and processed in such a way as to encode them with information. In practical terms, this means that when information-bearing waves are transmitted, a suitable device (often referred to as a “radio”), properly tuned, may intercept and decode them—gleaning in the process the transmitted information.
Information encoded and transmitted in radio wave may be used for many purposes. In effect, the information is simply a set of instructions to the receiver, which it will execute to produce a desired result. This result may be simply the reproduction of a sound, or involve something more elaborate such as piction, motion picture, or other visual display. It may even include a computer program for execution by the receiver. And of course it may include a combination of these effects. Naturally, for the intended effect to obtain, the receiver must be capable of receiving and processing the radio-signal borne instructions appropriately.
Radio communication was first put to use for two-way voice communication, but was soon adapted for broadcast use. Broadcast simply means that the program content (that is, the desired effect or presentation) is transmitted, usually on a relatively-powerful radio-wave signal, with the intent that it will be received by a large number of receivers. By using different frequencies (or frequency bands), numerous broadcasters can send their programs simultaneously. To take advantage of this multichannel programming, receivers (such as radios and televisions) are selectively “tuned” to receive, process and display only one broadcast transmission at a time. “Multicasting” is similar to broadcasting, but uses techniques such as encryption and coding to ensure that only a selected group of all the otherwise capable receivers will actually be able to receive and process the signals. The advantage of multicasting or broadcasting, obviously, is that a particular program needs only to be transmitted once in order to reach many subscribers. (Note that for convenience herein, the term “broadcast” will include “multicast”, unless in a particular instance its exclusion is manifest.)
More recently, radio telephony has gained in popularity, due in large part to technological advancements that both make it economically feasible for a large population, and also permit its widespread use even in crowded urban areas. Telephone communication, of course, began with a wire-line network that connected a number of telephone-service subscribers. To eliminate the need to connect each subscriber to every other one, switching offices were introduced. In a switching office, connections are made to enable the temporary creation of a complete electrical circuit between one caller and another. Each subscriber is connected to a local switching office by a set of wires, and can be connected to the local switching office of another through a series of connections that are set up temporarily for a particular cell. The switches, wires, and cables used to establish these circuits are captured only for the duration of the call, and are afterwards released for use by others.
Radio telephony must operate somewhat differently. While the network may be largely made of (automated) call-routing switches that are connected to each other by wire, communication between each individual telephone and the network are accomplished using radio communications. As should be apparent, however, there may be a great number of radio telecommunication subscribers operating in a given area, and using ordinary two-way radio communication they would soon interfere so frequently with each other's transmissions that the system would become unusable. The concept of cellular telephony and various frequency sharing techniques are used to avoid this condition.
As has been mentioned, calls by radio-telephone subscribers are routed through a network. The telephones, commonly referred to as mobile stations (MSs) because they can be used from any location within the network coverage area, communicate with a nearby base station (BS), which is in turn connected to the network. The network is divided into numerous “cells”, each having one or more base stations for communication with mobile stations located there.
For example, FIG. 1 is a simplified block diagram illustrating the configuration of a typical PLMN 100. As mentioned previously, the entire geographic area covered by PLMN 100 (which is not entirely shown in FIG. 1) is divided into a number of cells, such as cells 10 through 15 delineated by broken lines in FIG. 1. Although only six cells are shown, there are typically a great many. In the illustrated embodiment, each cell has associated with it a base transceiver station (BTS) for example BTS 20 for transmitting and receiving messages to and from mobile stations (MS) in cell 10, here MS 31, MS 32, and MS 33, via radio frequency (RF) links 35, 36, and 37, respectively. Mobile stations MS 31 through MS 33 are usually (though not necessarily) mobile, and free to move in and out of cell 10. Radio links 35–37 are therefore established only where necessary for communication. When the need for a particular radio link no longer exists, the associated radio channels are freed for use in other communications. (Certain channels, however, are dedicated for beacon transmissions and are therefore in continuous use.) BTS 21 through BTS 25, located in cell 11 through cell 15, respectively, are similarly equipped to establish radio contact with mobile stations in the cells they cover.
BTS 20, BTS 21, and BTS 22 operate under the direction of a base station controller (BSC) 26, which also manages communication with the remainder of PLMN 100. Similarly, BTS 23, BTS 24, and BTS 25 are controlled by BSC 27. In the PLMN 100 of FIG. 1, BSC 26 and 27 are directly connected and may therefore both communicate and switch calls directly with each other. Not all BSCs in PLMN 100 are so connected, however, and must therefore communicate through a central switch. To this end, BSC 20 is in communication with mobile switching center MSC 29. MSC 29 is operable to route communication traffic throughout PLMN 100 by sending it to other BSCs with which it is in communication, or to another MSC (not shown) of PLMN 100.
Where appropriate, MSC 29 may also have the capability to route traffic to other networks, such as a packet-data network 50. Packet-data network 50 may be the Internet, an intranet, a local area network (LAN), or any of numerous other communication networks that transfer data via a packet-switching protocol. Data passing from one network to another will typically though not necessarily pass through some type of gateway 49, which not only provides a connection, but converts the data from one format to another, as appropriate. Note that packet-data network 50 is typically connected to the MSC 29, as shown here, for low-data-rate applications. Where higher data rates are needed, such as in 3G CDMA networks (explained below), the packet-data network 50 may be connected to PLMN 100 differently (see for example FIG. 2).
There are distinct advantages to the cellular architecture. Because any given mobile station always communicates only with nearby base stations, lower transmission power may be used. In addition, the frequency bands used to define separate channels for use by each active subscriber in one cell may also be used in another relatively-nearby (though not neighboring) cell without concern for interference. Note that a particular frequency channel (or channels) is defined for use by an active subscriber, and when the call is done it may be released for use by another subscriber in the same cell. Mobile stations, which can communicate on many different frequencies within a designated range, are not permanently assigned a single frequency. Mobile stations are, of course, mobile, and various strategies have been developed to assign and un-assign channels for communicating with a particular base station as the mobile station moves in and out of its coverage area (preferably without call interruption).
In addition, frequency-sharing schemes have been developed so that a number of subscribers may share a frequency even if they are located in the same cell. In other words, a single frequency band may define a number of channels. One frequency-sharing scheme takes advantage of the fact that typical voice communications do not require constant transmission to be effective, and so divides a given frequency into numerous channels using time slots. A time slot is a short, recurring span of time that is assigned to each channel, and information on that channel is transmitted only within the assigned slot. This technique is referred to as time-division multiple access (TDMA). In TDMA, the time slot allocation of each frequency is made such that subscribers in a telephone conversation perceive no discontinuity.
Another multiple-access (frequency-sharing) scheme is referred to as code-division multiple access (CDMA). In CDMA, separate channels are formed by processing information signals for transmission using one of a defined set of codes. Communication between a base station and a mobile station coordinate which code or codes will apply to a particular call involving the mobile station. The codes are mutually orthogonal, so transmissions processed with them do not normally interfere with each other.
In order to make sure the various components of a network work properly together, various standard protocols are promulgated. Current CDMA equipment, for example, operates according to either the IS-95, or the more recent CDMA 2000 standard. (New equipment may work according to both, backward-compatibility being a desirable feature). The CDMA 2000 (also known as IS-2000) standard was developed in part, to accommodate the efficient transmission over the cellular air interface of non-voice content, such as data and streaming multimedia presentations.
This non-voice content presents its own transmission concerns; data, for example, although it can be sent in short, discontinuous bursts, must be virtually free from error (whereas a voice conversation can be understandable despite a relatively high number of transmission errors). Multimedia must not only be continuous and presented in proper order, it must be relatively error-free as well. Equipment that can effectively handle all of these types of transmissions is sometimes referred to as third-generation (3G) equipment; CDMA 2000 is a 3G standard.
As should be apparent, 3G communications require high transmission capacity and place a greater demand on network resources. In some transmission media, such as optical fiber, this poses little concern. In cellular radio, however, widespread use and the medium's physical limitations mean that radio channels may be severely taxed, and other techniques are needed to ensure they are efficiently utilized. One such technique is called compression.
Compression is a widely-used technique for the efficient storage and transmission of data. There are actually many such techniques used for a variety of techniques used for a variety of applications, and each has their own advantages and disadvantages. In general, however, these techniques rely on the ability of a compressor to represent a frequently-occurring block of data in an abbreviated form that will be understood by a decompressor, which reconverts the abbreviation to its original form. Naturally, the compressor is associated with a transmitting device and the decompressor with a receiving devices. Many devices, of course, both transmit and receive and accordingly also include both a compressor and a decompressor.
Data is frequently transmitted in packets. A packet is a discrete block of data that is part of a larger data set that has been broken up for efficient transmission. Packets may vary in size, according to the system through which they will travel, but their size is generally determined by the particular transmission protocol rather than by the specific content being transmitted. (Although different types of content may invoke the use of different protocols.) Data traveling in packets may be compressed, at least to some extent.
Packet data must be addressed. Packet-data systems, for example the Internet, do not establish a single “circuit” for transmission. Rather, each packet is routed to its destination through whatever route is most easily available. The individual packets related to a particular block of information may therefore take different routes to their destination, where they will have to be reassembled in their original order to make sense. The receiver is informed what to expect and if any packets are lost in transmission, a retransmission request is generated. As should be apparent, each packet must have associated information identifying it and its destination. Packets therefore have in addition to their information content, or payload section, a “header” containing overhead information so that they may be properly routed and later reassembled in the correct order.
Naturally, the headers are added for transmission and discarded when they are no longer needed. Packet headers, however, must still be transmitted and received—and therefore consume network resources just as does the payload information itself. Header compression techniques are therefore applied in an attempt to conserve those resources—especially when the packet information is transmitted over the air interface. This is particularly important when broadcasting multimedia content, which in the CDMA 2000 context is typically sent according to the realtime transport protocol (RTP). In RTP, for example, if header compression is used on packets carrying encrypted broadcast content, the Internet Protocol (IP) header may be compressed, as may the security parameter index (SPI) field of the encryption security payload (ESP) header. When this compression is used, the overhead due to transport and encryption of the broadcast content is reduced by approximately thirty percent.
Before a mobile station can decompress received packet headers, however, the context of its decompressor needs to be synchronized with the transmitting node's compressor. For this to occur, the full (uncompressed) header needs to be transmitted (and received), often more than once. And to prevent delay and minimize tuning time, the full header must be transmitted as soon as possible. Not being compressed, however, the full header uses up valuable (radio) broadcast channel resources. In the broadcast scenario, moreover, the same content is typically being transmitted to a large number of receiving stations, each of which must be individually synchronized. Sending the full header frequently enough to ensure that they are all maintained in this condition detracts from the network's ability to take advantage of header compression techniques. Needed then, is a way for allowing many mobile stations to easily synchronize their packet-header decompressors without overtaxing the air interface. The present invention provides just such a solution.